linux-ext4 - Re: [PATCH 7/8] nowait aio: xfs

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Date:   Fri, 7 Apr 2017 08:08:51 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, jack@...e.com,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        sagi@...mberg.me, avi@...lladb.com, axboe@...nel.dk,
        linux-api@...r.kernel.org, willy@...radead.org,
        tom.leiming@...il.com, Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 7/8] nowait aio: xfs

On Fri, Apr 07, 2017 at 06:34:28AM -0500, Goldwyn Rodrigues wrote:
> 
> 
> On 04/06/2017 05:54 PM, Darrick J. Wong wrote:
> > On Mon, Apr 03, 2017 at 11:52:11PM -0700, Christoph Hellwig wrote:
> >>> +	if (unaligned_io) {
> >>> +		/* If we are going to wait for other DIO to finish, bail */
> >>> +		if ((iocb->ki_flags & IOCB_NOWAIT) &&
> >>> +		     atomic_read(&inode->i_dio_count))
> >>> +			return -EAGAIN;
> >>>  		inode_dio_wait(inode);
> >>
> >> This checks i_dio_count twice in the nowait case, I think it should be:
> >>
> >> 	if (iocb->ki_flags & IOCB_NOWAIT) {
> >> 		if (atomic_read(&inode->i_dio_count))
> >> 			return -EAGAIN;
> >> 	} else {
> >> 		inode_dio_wait(inode);
> >> 	}
> >>
> >>>  	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
> >>>  		if (flags & IOMAP_DIRECT) {
> >>> +			/* A reflinked inode will result in CoW alloc */
> >>> +			if (flags & IOMAP_NOWAIT) {
> >>> +				error = -EAGAIN;
> >>> +				goto out_unlock;
> >>> +			}
> >>
> >> This is a bit pessimistic - just because the inode has any shared
> >> extents we could still write into unshared ones.  For now I think this
> >> pessimistic check is fine, but the comment should be corrected.
> > 
> > Consider what happens in both _reflink_{allocate,reserve}_cow.  If there
> > is already an existing reservation in the CoW fork then we'll have to
> > CoW and therefore can't satisfy the NOWAIT flag.  If there isn't already
> > anything in the CoW fork, then we have to see if there are shared blocks
> > by calling _reflink_trim_around_shared.  That performs a refcountbt
> > lookup, which involves locking the AGF, so we also can't satisfy NOWAIT.
> > 
> > IOWs, I think this hunk has to move outside the IOMAP_DIRECT check to
> > cover both write-to-reflinked-file cases.
> > 
> 
> IOMAP_NOWAIT is set only with IOMAP_DIRECT since the nowait feature is
> for direct-IO only. This is checked early on, when we are checking for

Ah, ok.  Disregard what I said about moving it then.

--D

> user-passed flags, and if not, -EINVAL is returned.
> 
> 
> -- 
> Goldwyn